Devices utilized in dermatological treatments often incorporate light based energy sources or high frequency rf electrical energy sources. Examples of such devices are described in U.S. Pat. No. 6,511,475. Some devices include both technologies.
A. Lasers and Light-Based Technologies
Lasers and light-based devices have been used for many years in the treatment of dermatological conditions. Soon after the laser was invented in 1957, medical researchers started to explore its use for a wide range of dermatological procedures. In recent years, especially since the mid-90's, the technology has been commercialized into numerous different devices that remove unwanted hair, wrinkles, fine lines and various facial blemishes (“skin rejuvenation”), tattoos, and vascular and pigmented lesions. Because of the short treatment time, virtually no patient “down-time” and fewer side effects, several of these laser- or light-based treatments have become more widely used than the conventional alternatives.
Light energy, when applied directly to the human body, is absorbed by the target chromophore; by the hemoglobin in the blood; the water in the skin; the melanin in the skin; and/or by the melanin in the hair follicles, depending on the wavelength(s) of the light used. Lasers generating different wavelengths of light were found early on to have different properties, each being preferable for specific procedures. In addition to lasers that emit a coherent, monochromatic light, several manufacturers have also introduced devices that emit light of a wide range of wavelengths that practitioners then filter to select the appropriate wavelength for a specific treatment. These “multi-wavelength” or “multi-application” light-based devices have the advantage of performing several different aesthetic treatments, and thus costing the practitioner less than purchasing several lasers individually.
FIG. 1a is a diagram showing the various layers of the skin and potential targets for photo therapy and/or electrical therapy. When light energy first impacts the skin, it encounters the epidermis, the outer most layer of skin. One of the substances that comprise the epidermis is melanin, the brown pigmentation that most of us have in our skin. Darker individuals have more melanin than lighter ones. For very dark individuals, melanin may comprise more than 20% of the epidermis. For light skin individuals, melanin may comprise only 1 to 2% of the epidermis.
Melanocytes in the upper epidermis generate this melanin in response to sunlight. The melanin migrates from the cell and forms a protective umbrella over the fibroblasts and other cells in the skin. The melanin absorbs harmful UVA and UVB radiation that can cause cell damage. It also absorbs visible light, absorbing blue light more than red light.
The epidermis is very thin as it is only 50 to 100 microns in thickness. Consequently, despite the strong absorption by melanin, a reasonable percentage of the light passes through the epidermis into the upper layer of the dermis. For a fair skin person, as little as 15% of the light in the visible portion of the spectrum is absorbed in the epidermis. For a darker person, the percentage absorbed can be more than 50%.
After passing through the epidermis, the light impacts a region called the dermal plexus. This is a thin region at the outer most region of the dermis. It contains a high concentration of small capillary vessels that provide nourishment to the overlying epidermis. The blood in these vessels absorbs between 35% and 40% of the visible portion of the light that impacted the skin.
Clearly for a moderate to dark skin individual, the majority of the visible portion of the spectrum is absorbed in the epidermis and the dermal plexus. Very little energy remains to treat a target located deeper than the dermal plexus.
FIG. 1b shows the percentage of incident energy transmitted, as a function of wavelength, through the epidermis for three different skin types. The figure shows a low percentage of the incident energy in the visible portion of the spectrum is transmitted through the epidermis. The energy not transmitted is absorbed, resulting in a rise in temperature of the epidermis and possibly resulting in the burning of the tissue.
FIG. 1c shows the percentage of incident energy transmitted through the dermal plexus for two different levels of blood concentration (shown as ratios of blood to the rest of the tissue in a given volume). As in the epidermis, the energy not transmitted is absorbed and can produce burning. More importantly, the energy absorbed in the dermal plexus is not available to heat a target such as collagen or tattoo ink that is located beneath the dermal plexus. By reducing the concentration in half, the energy transmitted is doubled.
B. High Frequency rf Electrical Devices
In addition to light based therapies, high frequency rf electrical energy is also becoming common in devices used to treat wrinkles, unwanted hair and unwanted vascular lesions. One of the basic principles of electricity is an electric current passing through a resistive element generates heat in that element. The power dissipated in the element is proportional to the square of the electrical current and also proportional to the resistance of the element. The heat generated is the product of the power times the length of time the power is being dissipated.
A second basic principle of electricity is the electric current seeks the path of least resistance. If two or more such paths exist, the current divides itself proportionally to the resistance of each path. For example, if two such paths exist and one path is twice the resistance of the other, twice the current will pass through the path with the lesser resistance than passes through the path with more resistance. The distribution of power and energy is also in the ratio of the resistances. In the current example, two times the power is dissipated in the lower resistance path than in the higher path. The path with the lesser resistance will heat at twice the rate as the higher resistance path.
High frequency rf energy in dermatology works on the principles described above. In this case, the various tissues and components of the body are the electrical resistors. As the rf current passes through these tissues, energy is dissipated and the temperature of the tissue rises. If the tissue is a blood vessel, it may reach a temperature at which the blood denatures and coagulates. If the tissue is collagen, it may reach a temperature at which the collagen denatures and is destroyed. The body's natural immune system removes the destroyed tissue, starting a process to regenerate new tissue.
The electrical resistance of various tissues varies widely. Tissues in the body with relatively high resistance are bone, fat and the outer layer of the epidermis. Tissues with moderate resistance are connective tissue and the dermis. The tissue with the lowest resistance is the blood. When high frequency electricity is used in dermatological applications, it tends to follow the pathways of the blood vessels, avoiding the fatty tissues and connective tissues.